It is well known that over extended periods of storage diorganopolysiloxane compositions may discolor. An example of such a diorganopolysiloxane composition comprises an alkoxysilyl terminated diorganopolysiloxane and an alkoxysilane. Such compositions are generally cured at room temperature by de-alcoholation in the presence of a suitable catalyst (e.g. a titanium compound). Since this composition is odor-free and does not cause corrosion of metals, it is used as an adhesive agent and sealant in the electrical, electronic, and construction industries. However, after storage for extended periods of time, this composition is subject to yellowing, the intensity of which increases with time.
The reason why yellowing of the aforementioned diorganopolysiloxane composition occurs is not completely clear, but it may generally be assumed that one of the main causes of this phenomenon is the presence of an organo-titanium compound used as the curing catalyst. Other factors which cause discoloration may include the use of additives such as amine-containing silane coupling agents or the like.
If the diorganopolysiloxane composition has changed its color during storage, products obtained from such composition will also change their appearance. Therefore, a demand exists for diorganopolysiloxane compositions that will substantially not change color during storage or only change color to an insignificant degree. Change of color becomes especially visible in various white and light-color sealants, which are most frequently used and therefore are in need of discoloration-resistant properties.
Japanese Laid-Open Patent Application Publication (hereinafter referred to as Kokai) S58-71951 (equivalent to EP0076330) discloses the elimination or decrease in discoloration of diorganopolysiloxane compositions of various types by means of the introduction of an ester of a thioalkane acid, a di-tertbutylphenol-containing compound, and/or an organic titanium compound chelated with an organic phosphite.
Kokai H9-3330 (equivalent to EP0747443) discloses the prevention of discoloration in a diorganopolysiloxane composition by utilizing an organo-titanium compound of a specific molecular structure.
Disadvantages of the above-described prior-art techniques include the facts that they are only efficient against discoloration due to the effects from organo-titanium compounds and that these methods are costly.
Kokai H9-087611 describes a yellowing-proof silicone sealant comprising a polyether-alumina-treated titanium oxide pigment and a hindered amine photostabilizer. Zinc pyrithione is used as an anti-fungal agent. This document teaches that, without the presence of the hindered amine stabilizer, the antifungal agent (zinc pyrithione, otherwise known as zinc (2-pyridylthio-1-oxide)) will deteriorate when subjected to ultra violet light resulting in an increased tendency for the sealant to yellow and for a loss of gloss on the surface, both due to the bleeding out of the antifungal agent from the sealant. The presence of the hindered amine stabilizer in the sealant is said to prevent the decomposition of the fungicide.